1. Field of the Invention
The present invention relates to an opto-electric hybrid board being a laminate made up of an electric circuit board having an element such as an optical element mounted and an optical waveguide, and to a method of manufacturing the same.
2. Description of the Related Art
In recent electronic devices and the like, optical interconnection in addition to electrical interconnection has been used so as to address an increase in the amount of transmission information. An exemplary technique may be an opto-electric hybrid board as shown in FIG. 18 (for example, see Japanese Laid-open Patent Application Publication No. 2011-048150). The opto-electric hybrid board includes: a flexible circuit board E0 including a flexible board 51 provided with an electrical interconnect line 52 on its front surface; and an optical waveguide (optical interconnection) W0 (including an under cladding layer 56, a core 57 and an over cladding layer 58) made of epoxy resin or the like, the optical waveguide W0 being stacked on the back surface (the surface opposite to the surface where the electrical interconnect line 52 is formed) of the flexible board 51. An optical element 4 is mounted on the flexible circuit board E0. Owing to the small thickness of both the flexible circuit board E0 and the optical waveguide W0, the opto-electric hybrid board is flexible. Therefore, dealing with the recent trend toward miniaturization of the electronic devices and the like, the opto-electric hybrid board is suitable for use in a small space as being bent, or in a movable section such as a hinge.
However, because of the flexibility, when the optical element 4 is mounted on the flexible circuit board E0, both the flexible circuit board E0 and the optical waveguide W0 deform under the pressure load caused by the mounting work. Accordingly, accurate mounting is difficult to be achieved, i.e., mounting workability is poor.
On the other hand, another disclosed opto-electric hybrid board further includes, as shown in FIG. 19, a stainless steel layer M0 formed across the entire opto-electric hybrid board between the flexible circuit board E0 and the optical waveguide W0 (for example, see Japanese Laid-open Patent Application Publication No. 2009-265342). The opto-electric hybrid board exhibits excellent mounting workability with the optical element 4 since the stainless steel layer M0 functions as reinforcement to suppress deformation under the pressure load caused by the mounting work with the optical element 4.
However, since the stainless steel layer M0 is provided across the entire opto-electric hybrid board, the opto-electric hybrid board lacks flexibility. Therefore, the opto-electric hybrid board is not suitable for use in a small space as being bent, or for use in a movable section such as a hinge.
Accordingly, what is also disclosed is an opto-electric hybrid board in which part of the over cladding layer of the optical waveguide is increased in thickness. The part corresponds to the mounting position of the optical element 4 where flexibility is not required (for example, see Japanese Laid-open Patent Application Publication Nos. 2010-286674 and 2011-017993). With the opto-electric hybrid board, numerous dummy cores not used for optical communication are formed in the region on the front surface of the under cladding layer, which region corresponds to the part of the over cladding layer to be increased in thickness. Thus, the part of the over cladding layer covering the dummy core region becomes thick. This thickness suppresses deformation under the pressure load caused by the mounting work of the optical element 4, thereby improving mounting workability with the optical element. However, such practice invites the following problem in manufacturing.
Generally, an element such as the optical element 4 is mounted on the opto-electric hybrid board by the ultrasonic mounting process. The ultrasonic mounting process is performed as follows. In a state where the opto-electric hybrid board is placed on a stage S with the optical waveguide W0 being on the bottom side as shown in FIG. 20, the optical element 4 attached to the tip of a nozzle N is pressed from above against electrodes 52a formed at the front surface of the flexible circuit board E0 which is on the top side, and vibrated at the ultrasonic frequency in the direction being parallel (see arrow X in FIG. 20) to the plane being perpendicular to the pressing direction (see arrow Z in FIG. 20). Thus, the electrodes 52a of the flexible circuit board E0 and gold bumps (not shown) formed on the surface of electrodes 4a of the optical element 4 are rubbed against each other, such that the electrodes 52a of the flexible circuit board E0 and the gold bumps of the optical element 4 are fused by the frictional heat to be joined to each other.
However, though both the conventional opto-electric hybrid boards, i.e., the one including the stainless steel layer M0 between the flexible circuit board E0 and the optical waveguide W0 (Japanese Laid-open Patent Application Publication No. 2009-265342) and the other one in which the over cladding layer is thickly formed at the part corresponding to the portion where the optical element 4 is mounted (Japanese Laid-open Patent Application Publication Nos. 2010-286674 and 2011-017993), can suppress deformation of the opto-electric hybrid board attributed to the load pressing the optical element 4, the optical waveguide W0 being in contact with the stage vibrates in accordance with the ultrasonic vibrations and hence deforms (see the dashed-two dotted line in FIG. 20). Thus, friction between the electrodes 52a of the flexible circuit board E0 and the gold bumps of the optical element 4 becomes insufficient to generate full frictional heat, making it difficult for the electrodes 52a of the flexible circuit board E0 and the gold bumps of the optical element 4 to join to each other. In particular, since those with a great thickness (Japanese Laid-open Patent Application Publication Nos. 2010-286674 and 2011-017993) tend to easily deform by the ultrasonic vibrations, it is more difficult to achieve joining.